Foam sandwich dinghy, laminate schedule?

I'm aware of similar threads on this forum but my project has very different engineering challenges so I'm starting a new thread.

I'd like to build a divinycell foam-cored fiberglass dinghy similar in design to the new OC Tenders made in New Zealand (which undoubtedly are fantastic boats, but would cost ~$8000 USD by the time they are shipped to NE USA).

Dimensions:
LOA- 8.8'
Beam- 5.1'
LWL- 8.6'
Draft- 0.75'

OC Tender claims the weight of their OC270 model (with similar dimensions above) at 77 lbs for the fiberglass version and 64 lbs for a carbon fiber version. I'm not an engineer but to achieve such weights from my calculations, there is no way they are using more than a single layer of reinforcement material on each side of the foam core. So I'm assuming a lot of the hull's rigidity comes from its many angles and the partial deck that acts to stiffen the upper part of the hull.

My goal is to replicate the lay-up as much as possible to achieve an ultralight dinghy that is still capable of daily use.

My guess is at these weights, there isn't CSM anywhere on the boat as this would add substantial weight without any real structural support. Biaxial, or triaxial (45/-45, 0) seem like the only material that could provide enough strength in multiple directions while still keeping the weight down.

So, given the intended use (daily tender while cruising), I'm wondering what the engineers (or those experienced with laminate schedules) think about such a design. Is it possible to emulate? I've attached a table with 3 different laminate options and the weight associated with each. I would be doing all of this by hand layup because vacuum bagging/infusion would make this a less cost-effective project than it's worth. I'm not so concerned matching the weight of a manufactured product but would still like to keep the dinghy as light as possible (maybe <90 lbs).

The table has formulas for everything so anyone is welcome to manipulate for their own uses. It assumes a wet layup cloth:resin ratio of 45:55 by weight.

TLDR:
Building a PVC foam core tender. Considering different cloth materials for the outer skins. All options would be a single layer of fiberglass on each side of the foam;

Attached Files:

I’m really interested in what you’re doing since I’m doing something similar. I’m building an ultra lightweight 12’ catamaran this summer. The OC tender is inspiring, and I hope you keep us up to date with your findings and build.

The pictures and videos of the OC tender shows foam panels as a rub rail or fender. From the light weights they’re claiming I’m assuming the outer hulls are somewhat delicate. Also in their brochures they talk about how easy it is to repair nicks and dings.

The old saying goes if you build it and it breaks you built it too light and if it doesn’t then you built it too heavy

At this point my outer hulls side will be made from 27 oz of carbon/epoxy. That will be just over a 1/2 lb per square foot. You didn’t mention carbon, but perhaps that’s an option for you.

I’m really interested in what you’re doing since I’m doing something similar. I’m building an ultra lightweight 12’ catamaran this summer. The OC tender is inspiring, and I hope you keep us up to date with your findings and build.

The pictures and videos of the OC tender shows foam panels as a rub rail or fender. From the light weights they’re claiming I’m assuming the outer hulls are somewhat delicate. Also in their brochures they talk about how easy it is to repair nicks and dings.

The old saying goes if you build it and it breaks you built it too light and if it doesn’t then you built it too heavy

At this point my outer hulls side will be made from 27 oz of carbon/epoxy. That will be just over a 1/2 lb per square foot. You didn’t mention carbon, but perhaps that’s an option for you.

Rob

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Rob, good luck with your build. Are you documenting this project somewhere that I can follow along?

Yes, I'm assuming at these weights, this will not be a beater and will require some tender treatment. I like the foam rubrail they've implemented and think it belongs on any GRP tender not only to protect itself but also the mothership. I'm planning on incorporating this element into my build as well. Carefully cut pool noodles and a Sunbrella pouch should work nicely at a very low cost.

How did you decide on that combination/weight of reinforcement?

Certainly, even a very lightweight layer of carbon (~6 oz) would add tremendous strength (impact resistance and rigidity) but would push this project far over my budget. Perhaps I should've mentioned earlier. The 3 options I've listed in my table would each cost between $1100 & $1500 USD at the time of writing. Probably another $500-600 in miscellaneous building supplies (peel ply, plywood for mold frames, sanding discs, brushes, rollers, etc.). If this build enters the $2000+ range, then I would probably just consider an aluminum RIB, despite the downsides to inflatables.

Just a few random thoughts - I am not trying to be objectionable, far from it.

The OC dinghies do look very impressive, but is your Option 1 really a 'lightweight' at 80 lbs (never mind Option 2 at 105 lbs)?
The 'deck' will add a fair bit of additional weight (although it will also stiffen the hull up considerably)

Even in this carbon age, plywood still has a lot going for it - a 9' long Catspaw plywood dinghy also weighs about 80 lbs (but with 9" less beam) -Catspaw / Two-Paw Plans https://bandbyachtdesigns.com/catsaw-two-paw/
And this is just plywood and epoxy / glass sheathed.
OK, I appreciate that you cannot put a (relatively) big outboard engine on the back of a Catspaw (or similar) to get her up on the plane (they recommend 2.5 hp maximum) - but she would be easy (and good fun) to row.
I have a feeling that your dinghy will have quite a lot of form resistance due to it's shape, and it will require a fair bit more effort to row than something like a Catspaw.
But if you need to go places fast, in similar fashion to an aluminium RIB, then this would be the compromise that you would have to make.
If you introduce another chine, you could probably build a dinghy like this from single skin fibreglass at less cost while still keeping the weight down because the panel sizes are smaller (?)

Also 15 pounds of gelcoat? Why would you even consider a 20% weight increase to gelcoat this thing?

And the bottom made with 1/4" core and a high resin ratio will not be very good. If you bump it; you will delam. The core for the bottom is better at 12mm. And you will experience less delam if you vac bag.

The motor also dictates things; the one I am building is rated for 3hp. I plan oars.

I redid your sheet with some changes. The 1/4" core is really insufficient in my opinion. You did not correctly calculate the core weights and were a bit high. Once I redid the sheet in 12oz biaxial; it became clear that even hand laminated with thicker cores, but reducing the resin rates to 1:1, using gelcoat becomes a 30% weight increase. My weights are a bit light because you need to add 12 oz tapes at all seams..didn't want to go that far.

Attached Files:

Rob, good luck with your build. Are you documenting this project somewhere that I can follow along?

Yes, I'm assuming at these weights, this will not be a beater and will require some tender treatment. I like the foam rubrail they've implemented and think it belongs on any GRP tender not only to protect itself but also the mothership. I'm planning on incorporating this element into my build as well. Carefully cut pool noodles and a Sunbrella pouch should work nicely at a very low cost.

How did you decide on that combination/weight of reinforcement?

Certainly, even a very lightweight layer of carbon (~6 oz) would add tremendous strength (impact resistance and rigidity) but would push this project far over my budget. Perhaps I should've mentioned earlier. The 3 options I've listed in my table would each cost between $1100 & $1500 USD at the time of writing. Probably another $500-600 in miscellaneous building supplies (peel ply, plywood for mold frames, sanding discs, brushes, rollers, etc.). If this build enters the $2000+ range, then I would probably just consider an aluminum RIB, despite the downsides to inflatables.

Click to expand...

I don’t have any sort of blog or anything, but my wife is threatening to take some videos and pictures.

I’ve done a bit of studying and reading, but I believe making your own test samples really gives you a feel for the materials.

I understand about the budget. I’m sure you’ve looked around for competitive pricing. You can save on the peel ply by going to a fabric store and buying the ladies dress lining. It’s around $2 a yard. Yes, I tested that too. The official peel ply was a little heavier but the stuff from the mill outlet worked perfectly at a fraction of the cost.

Hi! Carbon can be pretty cheap these days. Factor in that you'll need less weight and less epoxy. 9oz, 300g/sqm should be pointy. If you calculate 1' deck, 1' side, 2,5' wide bortom, each side. Times 10' length. Times 2 (inside and outside) you get about 16sqm of fabric Times 12.5€ or about 15USD= 240USD.
So i'm not sure what the fuss about expensive carbon is all about. More that carbon is slightly more difficult to wet out and to bend in sharp corners.Carbon fabrics https://www.fibermaxcomposites.com/shop/carbon-fabrics-c-36.html

Hi! Carbon can be pretty cheap these days. Factor in that you'll need less weight and less epoxy. 9oz, 300g/sqm should be pointy. If you calculate 1' deck, 1' side, 2,5' wide bortom, each side. Times 10' length. Times 2 (inside and outside) you get about 16sqm of fabric Times 12.5€ or about 15USD= 240USD.
So i'm not sure what the fuss about expensive carbon is all about. More that carbon is slightly more difficult to wet out and to bend in sharp corners.Carbon fabrics https://www.fibermaxcomposites.com/shop/carbon-fabrics-c-36.html

Hi! Carbon can be pretty cheap these days. Factor in that you'll need less weight and less epoxy. 9oz, 300g/sqm should be pointy. If you calculate 1' deck, 1' side, 2,5' wide bortom, each side. Times 10' length. Times 2 (inside and outside) you get about 16sqm of fabric Times 12.5€ or about 15USD= 240USD.
So i'm not sure what the fuss about expensive carbon is all about. More that carbon is slightly more difficult to wet out and to bend in sharp corners.Carbon fabrics https://www.fibermaxcomposites.com/shop/carbon-fabrics-c-36.html

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Carbon is light stiff and strong, But it is brittle. Using only 9 oz for even a hull side would be extremely light but it’s life would be extremely short . Three layers of the 9oz for a hull side is the answer for me

I redid your sheet with some changes. The 1/4" core is really insufficient in my opinion. You did not correctly calculate the core weights and were a bit high. Once I redid the sheet in 12oz biaxial; it became clear that even hand laminated with thicker cores, but reducing the resin rates to 1:1, using gelcoat becomes a 30% weight increase. My weights are a bit light because you need to add 12 oz tapes at all seams..didn't want to go that far.

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Fallguy, thanks for your suggestions and for adding those other options to the table. Just to clarify, our cubic foot calculations are equivalent, I just used 96 sqft for the hull instead of the designed 69 sqft in case I wanted to triple up the core thickness at the transom. I'd like to be able to use a 6hp outboard without the boat falling apart! Either, way the differences in core weight are trivial compared to the other materials needed.

Thanks for the suggestion of moving up to 1/2 inch core for the hull and 1/4 inch for the deck. By "cap" I assume you mean the deck. Again, I'm no engineer/naval architect but my understanding is that the deck will help provide stiffness to the hull. Additionally, it will increase the seating area and the area beneath can be used to hang some storage bags and keep contents out of contact with water (again, see the OC Tender).

The 45:55 cloth/resin ratio is the ballpark estimate I've read from manufacturer's websites and other forum posts when utilizing hand layup techniques. Yes, ideally this should be 1:1 but I've read that achieving this is only possible with vacuum bagging. And again, a vacuum bagging setup is out of my budget.

I am not at all in favor of adding 15 lbs of gelcoat (believe me, it sounded just as crazy in my mind). I was only trying to make a reasonable guess as to the weight of a finishing material that can stand up to UV and saltwater. I am totally open to other suggestions!!! On this note, should I also apply an epoxy barrier coat? Antifouling? The boat will spend most of its time in davits while not in use or on the foredeck underway.

Epoxy primers and two part paints are somewhat easy to apply and very durable.

the missing seats will provide more strength than a deck and weight not in the calcs; you can reinforce the sheer with an extra piece of core scrap with a bevel for glass transition..this also allows a 3/8 radius for glass wrapping the sheer

I generally use a wetout of 1:1 when laying up by hand. You can do better, but the core needs some resin. Familiarize yourself with a squeegee and learn how to apply resin beneath the glass to simplify wetting out. You basically use a roller to wetout the assembled core with about 40-60% of the resin needed and even things out and then even out more with squeegee.

12 oz glass is a litte snaky; so you will want to make sharpie references if you intend to wetout under it at all or you will stretch it to hell placing and replacing

I don’t have any sort of blog or anything, but my wife is threatening to take some videos and pictures.

I’ve done a bit of studying and reading, but I believe making your own test samples really gives you a feel for the materials.

I understand about the budget. I’m sure you’ve looked around for competitive pricing. You can save on the peel ply by going to a fabric store and buying the ladies dress lining. It’s around $2 a yard. Yes, I tested that too. The official peel ply was a little heavier but the stuff from the mill outlet worked perfectly at a fraction of the cost.

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